How can I append a group after having it linked earlier?
So here is a very simple solution of how to append a group after having previously linked it in or how to simply get rid of an unwanted group instance:


In the Outliner, switch to "Blender File", and open up "Libraries".





Right click the library referencing the linked group, and press "Delete".




You will now be able to immediately append the group as normal, no saving or re-opening required!